Fannie / Freddie set the limits for the high-balance or super confirming limits in the DC area, currently at $625,500. Financing above this number would entail a second mortgage to make up the difference or true jumbo (both not Fannie / Freddie), and usually portfolio / bank specific.
Many banks offer true jumbo products, terms and qualification is specific to each bank with different requirements. With 20% down you should have the best of both options. Our bank has a very good in-house second mortgage product, I'm happy to run numbers for you on the first and second split mortgage option.